This invention relates to communication systems of all types which employ amplitude-modulated (AM) carrier or subcarrier signals to convey information signals (such as speech or music audio) from a transmitter to a receiver. Such applications generally transmit the aforementioned information on radio-frequency carriers via either conductors or through the atmosphere to the receiving devices, which in turn decode the information and convert it to a signal essentially identical with the original version. The principal application of the new signal format would most likely be in FM (and potentially TV aural) broadcasting, where an amplitude modulated subcarrier is employed to handle the stereo difference-channel modulation which with the main (sum-channel) signal is used to regenerate the original left- and right-channel audio material.
The invention described is a specific form of vestigial-sideband AM (VSB) wave which possesses amplitude and spectral properties basically intermediate to those of the classic double-sideband (DSB) and single-sideband (SSB) types. Specifically, the double-sideband signal occupies twice the bandwidth as the original modulating signal, since it incorporates two sidebands placed symmetrically about the center or carrier frequency; the SSB wave has only one sideband (either above or below the carrier) and exhibits a bandwidth equal to that of the modulating signal. The DSB signal has an amplitude envelope proportional to the magnitude of the modulating waveform, whereas the amplitude of the SSB signal envelope is in general greater than that of the modulation. The relationship between the SSB wave and the original modulation is complex and is dependent on the nature of the modulating waveform; the peak value of the SSB signal will actually approach infinity for a true square-wave modulating signal. For the complex bandlimited waveforms typical of speech and music audio, the SSB wave can easily be 6 to 12 dB higher in peak amplitude than the original signal (or the DSB form); this 6-12 dB factor would require the SSB transmitter to be capable of up to four times the output signal modulation amplitude of an equivalent system employing conventional DSB techniques. To avoid this difficulty, the standard practice is to employ DSB and simply accept the increased bandwidth requirement. In some applications, however, it would be highly desirable to reduce the bandwidth of the AM signal to a value substantially less than the DSB form while maintaining peak signal amplitudes near the DSB value.
The proposed "special" VSB signal (referred to as SVSB) is specifically configured to achieve these goals and to simultaneously permit low-distortion reception via the conventional synchronous detection techniques employed for DSB systems. Although vestigial sideband transmission has been employed for many years in U.S. television broadcasting to send the visual information, the SVSB signal described herein is tailored to the needs of AM-based transmission and reception systems requiring a very high degree of accuracy in the demodulated signals, such as found in high-fidelity audio broadcasts.